This invention relates to error detection and correction, and more particularly to improved apparatus and methods for combining error syndromes and data for enhancing the eerror correcting procedures.
The addition of redundant bits to a data message for error correction and detection employing block codes is well known for correcting a plurality of symbols in error in a burst of errors. For example, in U.S. Pat. No. 3,629,824, encoding and decoding apparatus detects a plurality of symbols in error, can correct one symbol in error without auxiliary pointers, and can correct two symbols in errors with auxiliary pointers. Under certain circumstances, the number of auxiliary pointers may not always be two; that is, there may be three pointers or no pointers. In such a situation, the apparatus of U.S. Pat. No. 3,629,824 is incapable of correcting the error, even though errors have been detected. In accordance with other procedures beyond the scope of the present description, error recovery is attempted. This can be done in magnetic media by re-reading the record and, in data communication, by retransmission. In some instances, the re-read of magnetic media or retransmission may result in the same number of pointers; i.e., there may be no pointers or increase in pointers. Data is therefore still uncorrectable.
There is a finite possibility, when data is interleaved as described later in the specification, that the first byte of a plurality of code words constitutes a pointer window and the second byte of the same plurality of code words constitutes a second pointer window, etc., that the burst error may not coincide with the pointer window boundaries. In such a situation, delimiting the error burst can bring the error condition within the capability of the error correcting apparatus, even though the auxiliary pointers indicate otherwise.